Durable glass enamel compositions are known in the art. These glass (or ceramic) enamel compositions are known to be useful for forming decorative coatings for glassware, chinaware, architectural glass and the like. They are especially useful in forming colored borders around glass sheets used for automotive windshields, sidelights and backlights. These colored borders enhance the appearance as well as prevent UV degradation of underlying adhesives.
In general, these enamel compositions consist essentially of a glass frit, a colorant and an organic vehicle or carrier. They are applied to a desired substrate in a single layer and subsequently fired to burn off the organic vehicle and fuse the frit, thus bonding the enamel coating to the substrate. Problems and shortcomings associated with conventional coatings have been dealt with by adjusting the composition of the glass enamels. These adjustments to the composition alter the performance and usually increase the cost of the resulting glass enamels. Further, while these adjustments to the compositions may improve one performance characteristic, they can potentially negatively affect another, or otherwise increase material and/or production costs. Thus a balancing of needs, cost, and performance is required when altering the composition of traditional glass enamels.
For example, the use of enamel coatings may negatively impact mechanical properties of the substrate, such as strength and durability. A related problem occurs when increasing opacity of the coating. Opacity depends on the thickness of the enamel and the pigment loading therein. This need arises in the automotive glass, where a highly opaque border is coated to the glass in order to cover and protect underlying adhesive from UV degradation. High pigment loading compromises the strength and durability of the coating and consequently, a relatively large amount of enamel required, with a corresponding increase in cost.
Another problem arises when the substrate regions require different opacity, such as glass required to have a slightly opaque middle section and a dark border. To achieve this effect it is generally necessary to make at least two passes, one covering the entire substrate, and the second covering only the border in order to achieve a thicker coating along the periphery. This process is inefficient and costly.
A further problem with conventional coatings is associated with glass sheets for automotive use, where the glass is generally coated by screen printing with the ceramic enamel composition and then subjected to a forming process at elevated temperatures. During this treatment the enamel melts and fuses to the glass substrate and then the glass is formed into a desired final shape by forming dies. It is known that low viscosity coatings tended to adhere at high temperatures to the materials covering the forming die.
Various approaches have thus been suggested in order to facilitate the forming of glass sheets with ceramic enamel coated thereon in order to avoid the enamel adhering to the forming die. The goal of these various approaches was to increase the toleration of the enamels to the elevated temperatures for bending or forming, and to repeated contact between the glass sheet and the covered forming die. For example, the use of metal oxide powders, including bismuth oxide-containing formulations have been proposed. Certain enamels comprising oxidizable metals or other components which form crystalline phases cause weaknesses in the substrate and adverse changes in the mechanical properties of the automotive glass. Specifically, high thermal expansion systems produce a weak glass substrate, such as when lithium is present, and have poor silver bleed-through properties, as well as inadequate anti-stick properties.
As a further problem, a number of previous ceramic enamel systems employ a lead-containing glass frit. For environmental considerations it is desirable to avoid the use of any lead-containing system. Also, while some of the prior enamel systems may perform fairly well in conventional glass forming processes, some are not satisfactory for use in the newly-developed “deep bend” processes for forming highly curved automotive glass parts.
Various approaches have also been suggested in order to provide glass enamels with durable and adequate resistance to certain chemical agents, which they may contact. Certain components incorporated into an enamel to achieve these goals increase the cost of the enamels and can require higher firing temperatures for the enamels, thus increasing production costs. To this end, any pigment component included in the glass enamels must also be limited because increasing the pigment content of a glass enamel to produce vivid colors, decreases the durability of the enamel and increases the firing temperature. Also the bonding strength to the substrate will decrease as pigments are increased.